JOHN W. MITCHELL 147 



Tissue Composition 



After growth-regulating substances have been absorbed and moved 

 to the different parts of the plant they incite specific physiological 

 responses, and these are sharply reflected in or evidenced by the amount 

 and kind of chemical constituents in the plants. 



Tissues of stems that respond most easily to such substances as 2,4-D 

 are those that possess a high level of oxidation-reduction activity (18). 

 In bean plants these tissues are phloem, endodermis, cambium, and 

 xylem parenchyma. When 2,4-D, indoleacetic acid, or naphthaleneacetic 

 acid come in contact with these tissues a series of chemical changes is 

 generally set in motion. The end result of these responses depends in 

 part upon the amount and the kind of growth-regulating substances 

 used. If for example, a minute amount of 2,4-D is apphed to a sensitive 

 plant, then the chain of responses does not extend far and only those 

 reactions involved in cell elongation may be affected. If more of the 

 compound is applied the chain of responses may be carried on through 

 the process of cell division, the organization of these new cells into tissues, 

 and finally their orientation into organs such as roots. 



If we consider this series of responses from the chemical standpoint, 

 the first obvious effect is an increase in the water content of the cell 

 (17,32,7,45), which is paralleled by an increase in the size of the cell. 

 Brown (7) in testing bean plants found that after several days of treat- 

 ment the water content of leaf tissues was depressed by 2,4-D while that 

 of stem tissues was increased. Thus, the response by some leaf tissues may 

 differ from those of stem tissues with respect to the effect of 2,4-D on 

 their water relations. 



Two theories have been proposed to explain why stem tissues take 

 up water when treated with certain types of growth regulators. First, 

 the chemical may bring about the degradation of certain cell constituents 

 so that the osmotic pressure of the sap is increased (17). There is also 

 evidence that cells under the influence of indoleacetic acid, for example, 

 absorbtions, such as potassium, more readily than do untreated cells. 

 Both the absorption of ions and the degradation of cellular constituents 

 would tend to increase osmotic pressure and favor water uptake. 



The second theory deals with the effect of growth regulators on the 

 cell wall. The resistance of the wall to extension is thought to be lowered 

 by the chemical so that the cell enlarges, thus allowing absorption of 

 water until a new equihbrium is reached (45). 



